Compounds that are labeled or tagged have long been used in the chemical and biological sciences. Such compounds can be used in a variety of ways. For example, by labeling a compound that is known to be biologically active, one can more readily identify metabolites of the compound, one can determine the binding and/or receptor sites for the molecule, one can determine how long the compound remains in the body or other system, and so on.
One known way to label compounds is by attaching a dye marker to the compound. This is typically done by grafting a dye moiety onto the biologically active molecule or by incorporating the dye moiety into the biologically active molecule during its synthesis. It is important that the labeled compound retain the critical properties of the unlabeled compound such as selective binding to a receptor or nucleic acid, activation or inhibition of a particular enzyme, or ability to incorporate into a biological membrane. There are a wide variety of dye moieties available, including for example, dipyrrometheneboron difluoride dyes, fluorescein, fluorescein derivatives, rhodamine, rhodamine derivatives and Texas Red.
The imidazonaphthyridines, imidazopyridines and imidazoquinolines are part of a unique class of immune response modifier compounds that have the ability to induce the biosynthesis of interferon and other cytokines. See, for example, Gerster, U.S. Pat. No. 4,689,338; Gerster et al., U.S. Pat. No. 4,929,624; Gerster, U.S. Pat. No. 5,268,376; Gerster et al., U.S. Pat. No. 5,389,640; Nikolaides et al., U.S. Pat. No. 5,352,784; Lindstrom et al., U.S. Pat. No. 5,494,916; and International Publication WO 99/29693. Dyes, particularly fluorescent dyes, are typically relatively large, bulky molecules and it is possible that such a large substituent may impair the compound's ability to bind or otherwise interact with the subject cells in a manner that causes biologic response.